Understanding Which Version of Glucose Is Not Digestible by Humans

October 24, 2025 •

4 min read

While our bodies are built to efficiently process and absorb most starches and simple sugars, a specific polymer of glucose, known as cellulose, is not digestible by humans. This indigestible version of glucose, along with other types of resistant starches, is a key component of what we call dietary fiber and plays a critical role in gut health.

Quick Summary

This article explores the indigestible forms of glucose, primarily cellulose and resistant starch, explaining the biological reasons for our inability to break them down and highlighting their crucial health benefits.

Key Points

Cellulose is Indigestible: The most common form of indigestible glucose is cellulose, a structural component of plant cell walls that humans lack the enzymes to break down.
Beta-Glycosidic Bonds: The key reason for indigestibility is the beta-glycosidic linkage in cellulose and some starches, which human digestive enzymes cannot cleave.
Resistant Starch is Another Type: Resistant starch (RS) is a starch that escapes digestion in the small intestine and is categorized into several types based on its source and structure.
Gut Health Benefits: Indigestible glucose acts as a prebiotic, feeding beneficial gut bacteria and producing short-chain fatty acids that nourish colon cells.
Improved Blood Sugar Control: Resistant starch and fiber slow down the absorption of sugars, leading to better blood glucose and insulin regulation.
Dietary Fiber's Role: Both insoluble fiber (like cellulose) and soluble fiber (found in resistant starch) contribute to digestive health, increase stool bulk, and promote regularity.

The Indigestible Glucose: An Overview

Most people think of glucose as a simple sugar that our bodies absorb for energy. However, glucose is also the basic building block for more complex carbohydrates, or polysaccharides, and the way these units are linked together determines whether we can digest them. For humans, the key difference lies in the chemical bond. Our digestive enzymes, specifically amylase, are designed to break down alpha-glycosidic bonds, like those found in starch. However, we lack the necessary enzymes, such as cellulase, to break the beta-glycosidic bonds present in certain glucose polymers. This fundamental enzymatic difference is why some glucose-based compounds pass through our digestive system untouched, providing crucial bulk but no energy in the form of calories.

Cellulose: The Primary Indigestible Glucose Polymer

Cellulose is perhaps the most well-known example of a glucose polymer that is indigestible by humans. A major structural component of plant cell walls, cellulose is composed of long, straight chains of glucose units linked by beta-1,4 glycosidic bonds. This unique structure allows the chains to form strong, rigid fibers that give plants their strength. While herbivores like cows and termites have symbiotic bacteria in their guts that produce cellulase to break down these bonds, humans do not possess this capability. As a result, when we consume plant-based foods, the cellulose content passes through our small intestine without being broken down, acting as insoluble dietary fiber.

Common sources of cellulose include:

Whole grains
The skins of fruits and vegetables
Leafy green vegetables
Legumes
Nuts and seeds

Resistant Starch: Another Form of Indigestible Glucose

Another significant category of indigestible carbohydrates is resistant starch (RS). Unlike cellulose, which is inherently indigestible due to its bond type, resistant starch is a type of starch that physically resists digestion in the small intestine. It then travels to the large intestine, where it is fermented by gut bacteria, much like dietary fiber. Resistant starch can be classified into several types based on its source and structure, which influences its resistance to digestion.

The Five Types of Resistant Starch

RS1: Physically inaccessible starch found in seeds, legumes, and unprocessed whole grains where the starch is trapped within fibrous cell walls.
RS2: Naturally occurring, compact, and granular starch found in unripe bananas, raw potatoes, and high-amylose corn. Cooking often reduces its resistance, but cooling can promote retrogradation (the process of forming RS3).
RS3: Formed when starchy foods like potatoes, pasta, and rice are cooked and then cooled. This process, called retrogradation, rearranges the starch molecules into a structure that resists digestion.
RS4: Chemically modified starch created for industrial food purposes, often used in breads and cakes.
RS5: A starch-lipid complex formed by heating certain starches with fatty acids. This complex structure resists digestion.

Comparing Digestible and Indigestible Glucose


Feature	Digestible Starch (e.g., Amylose/Amylopectin)	Indigestible Carbohydrates (e.g., Cellulose/RS)
Glucose Linkage	Primarily alpha-glycosidic bonds	Primarily beta-glycosidic bonds
Enzymatic Action	Broken down by human enzymes like amylase	Not broken down by human enzymes
Metabolic Fate	Absorbed as glucose for energy in the small intestine	Passes to the large intestine for fermentation by bacteria
Nutritional Impact	Provides calories and energy	Provides fiber, promotes gut health, and has minimal calories
Physiological Effect	Rapidly absorbed; can spike blood sugar	Slows digestion, stabilizes blood sugar, increases satiety
Source Examples	White bread, baked potatoes, white rice	Whole grains, legumes, unripe bananas, cooled pasta

The Health Benefits of Indigestible Glucose

Despite not providing direct calories, indigestible glucose is essential for a healthy diet and confers several important health benefits. As dietary fiber, it acts as a prebiotic, serving as food for the beneficial bacteria in our large intestine. The fermentation of indigestible carbohydrates by these microbes produces short-chain fatty acids (SCFAs), such as butyrate, which nourish colon cells and are linked to a reduced risk of bowel disorders.

Additionally, indigestible carbohydrates help regulate blood sugar levels by slowing down the absorption of other carbohydrates, leading to a more gradual release of glucose into the bloodstream. This makes them valuable for managing insulin sensitivity and reducing the risk of type 2 diabetes. The bulking effect of insoluble fiber, such as cellulose, also promotes regular bowel movements and helps prevent constipation.

Conclusion: A Fiber-Rich Perspective on Glucose

The version of glucose that is not digestible by humans is primarily found in the forms of cellulose and resistant starch. This indigestibility is not a flaw in our biology but a crucial feature that supports overall health through the provision of dietary fiber. By understanding the different ways glucose can be structured, we can appreciate why not all carbohydrates are created equal and how consuming fiber-rich foods benefits our digestive system, manages blood sugar, and supports our gut microbiome. Incorporating a variety of indigestible carbohydrates from whole grains, legumes, and vegetables is key to a balanced diet and long-term wellness.

Visit the National Institutes of Health for more information on dietary fiber.

Frequently Asked Questions

Humans cannot digest cellulose because it contains beta-glycosidic bonds linking the glucose units. Our digestive system lacks the enzyme cellulase needed to break these specific bonds, unlike ruminant animals or certain gut microbes.

The main difference is the chemical structure of the glucose chains. Digestible starch contains alpha-glycosidic bonds that our enzymes can break, while indigestible starch (resistant starch) has a structure that physically resists enzymatic breakdown in the small intestine.

Consuming indigestible carbohydrates provides several health benefits, including improved gut health by nourishing beneficial bacteria, better blood sugar regulation, increased satiety, and improved bowel regularity.

Yes, resistant starch is found in many common foods. For example, unripe bananas and raw potatoes contain Type 2 RS, while cooked and cooled pasta, rice, and potatoes contain Type 3 RS.

Because indigestible carbohydrates are not broken down and absorbed in the small intestine, they provide minimal calories. Any energy derived comes from the short-chain fatty acids produced by gut bacteria during fermentation, which is far less than from digestible carbohydrates.

Indigestible carbohydrates contribute to satiety by slowing down the digestive process and increasing the bulk of food in the digestive tract. This helps you feel full for longer, potentially leading to lower overall calorie intake.

Instead of being digested in the small intestine, indigestible carbohydrates travel to the large intestine. There, they are fermented by gut microbiota, producing beneficial compounds like short-chain fatty acids, which are then absorbed.